1. Field of the Invention
The present invention relates to an integrated semiconductor light source which uses locking characteristics by external light injection and has an active region and a passive region.
2. Discussion of Related Art
Recently, demand for data service, for example, through the Internet is increasing, and due to new multimedia service such as an HDTV, an electronic commerce, a VOD, and a video conference, a need for sufficient data traffic is raised, so that severe research on a passive optical network (PON) which directly couples an optical fiber to respective subscribers from a central office (CO) is being performed.
In the PON, except for a transceiver, an outside network which couples the central office to the respective subscribers is comprised of passive optical elements such as an optical fiber, a distributor or a multiplexer/demultiplexer, and it does not need an additional power source and, thus there are advantages in that it is easy to maintain it and communication capacity can be increased or new communication service can be provided by just replacing a transceiving module. In the PON, a method that communication between the central office and the subscriber is performed using an intrinsic wavelength allocated to each subscriber is referred to as a wavelength division multiplexing passive optical network (WDM-PON).
The WDM-PON includes a distributed feedback laser diode (DFB-LD) array, a wavelength-tunable laser diode, a multi-wavelength laser diode, or a spectrum slicing type incoherent light source as an optical line terminal (OLT) light source which is responsible for data transmission from/to the central office.
The DFB-LD array, the wavelength-tunable laser diode and the multi-wavelength laser diode are ones which are integrated such that light of various wavelengths is emitted from one module and have advantages in that an oscillation characteristic is excellent, a side mode suppression ratio (SMSR) is high, and a spectral line width is narrow. But the DFB-LD array, the wavelength-tunable laser diode and the multi-wavelength laser diode have complicated manufacturing processes and so have high manufacturing cost. Because of such an economic reason, the spectrum slicing type incoherent light source attracts public attention.
The incoherent light sources such as a light emitting diode (LED), a super luminescent diode (SLD), and an amplified spontaneous emission (ASE) have a wide spectrum and are being employed in a wavelength division multiplexing optical transmission device as a method (i.e., spectrum splicing type) of obtaining a light source having several wavelengths by using an arrayed waveguide grating (AWG).
As an optical network terminal (ONT) light source, many suggestion and reports have been introduced, but a loop back method and an injection locking method are mainly employed. A method of generating a single mode optical output using an injection locking characteristic of a fabry-perot laser diode (FP-LD) will be described below for comparison with the structure of a light source of the present invention.
FIGS. 1A and 1B are views illustrating a cavity mode characteristic and a locking characteristic of injected light of a conventional FP-LD. As shown in FIG. 1A, the cavity modes of the laser are determined by an optical gain characteristic and a cavity length L of a laser material. Here, an interval between the cavity modes is Δλ, where λ is a wavelength, and ng is a group refractive index. Output on the respective cavity modes is in proportion to the magnitude of spontaneous emission coupled to the respective cavity modes.
In the characteristics of the laser diode described above, as shown in FIG. 1B, if light is artificially injected to a certain wavelength from an external portion, an optical output in the injected wavelength is increased. Here, an output in the rest wavelength other than the injected wavelength is decreased due to the injection locking characteristic, so that a single mode light source having the high SMSR can be obtained.
With respect to the wavelength of the injection light, the injection locking characteristic sensitively varies according to the cavity mode characteristic of the FP-LD, and thus it has the following problems.
Firstly, when the single mode light source of the injection locking method is implemented, the spectrum-sliced injection light is actually difficult to match the cavity mode of the laser. Thus, as shown in FIG. 2A, if the injection light is misaligned with the cavity mode wavelength (corresponding to a crest), the injection locking characteristic is significantly decreased.
Secondly, for the first reason described above, a structure of increasing mode selectivity for the injection light by increasing the cavity length to narrow the cavity mode interval can be used. However, if the spectrum width of the injection light is relatively greater than the cavity mode length as shown in FIG. 2B, the two modes beat against each other, so that it is impossible to obtain the single mode characteristic.
Thirdly, for the first reason described above, the cavity mode length can be adjusted by controlling temperature of the FP-LD with respect to the wavelength of the injection light. This method is disclosed in Korean Patent Application Nos. 10-2002-0057223 entitled “Temperature control method for automatical wavelength-locking of a Fabry-Perot laser diode to the wavelength of the injected incoherent light” and 10-2003-0008099 entitled “Wavelength-tunable light source and wavelength-division multiplexed transmission system with the sources”.
However, the method of turning the wavelength of the injection light by temperature adjustment needs high-price parts such as a thermo-electric cooler (TEC) and an automatic temperature controller (ATC), and thus cost is high. The method using the temperature adjustment is also slow in speed, and when temperature is increased, the optical gain of the FP-LD is decreased, whereby the optical output characteristic is degraded.
Fourthly, for the first reason described above, the wavelength of the injection light can be turned by adjusting an injection current of the FP-LD. This method is disclosed in Korean Patent Application No. 10-2003-0008099 entitled “Wavelength-tunable light source and wavelength-division multiplexed transmission system with the sources”.
However, if the injection current is adjusted in a case where the operating current of the FP-LD is lower than the threshold current, the optical output may be changed due to the variation of the optical gain. In a case where the operating current of the FP-LD is greater than the threshold current, even if the current is increased, since carrier density in the laser is barely changed, the refractive index is barely changed, whereby it is difficult to tune the wavelength.
For the foregoing reasons, the FP-LD using the injection locking characteristic has difficulties in employing as the ONT light source of the WDM-PON.
In the FP-LD of the injection locking method which is used as the ONT light source in the conventional WDM-PON, if it is put to practical use, since the wavelength of the injection light is difficult to exactly match the cavity mode of the laser, the injection locking characteristic is degraded, and even though the cavity length is increased to increase locking selectivity to the injection light, the two modes are beaten, thereby degrading the locking characteristic.
In the conventional art, the method of turning the wavelength of the injection light by adjusting the temperature or current of the FP-LD has been suggested, but it has disadvantages in that speed is slow and an optical output is greatly changed.